Ms Zatuliveter, 26, who was last month likened to alleged spy Anna Chapman during an immigration tribunal, is accused of passing secrets to Moscow when she was a researcher for Liberal Democrat MP Mike Hancock.

She was stopped at Gatwick Airport in August last year, and was arrested in December amid fears she was engaged in espionage. The government wants to deport her on the grounds that her presence is a danger to national security.

Mr Justice John Mitting, the chair of the Special Immigration Appeals Commission (Siac), said that “on the balance of probabilities” Katia Zatuliveter was not a Russian agent.

‘Vulnerable’

During the Siac tribunal, much of which was heard behind closed doors for reasons of national security, Jonathan Glasson, the lawyer acting on behalf of the home secretary, said Miss Zatuliveter targeted Mr Hancock, 65, because he was “potentially vulnerable”.

It reveals an extraordinarily poor investigative approach, more akin to Inspector Clouseau than George Smiley.Tim Owen QC

He told the hearing the Portsmouth South MP was known to have had a number of extra-marital affairs.

“You knew that Mr Hancock’s private life might make him potentially vulnerable,” he told the Russian in cross-examination.

The parliamentarian was also a member of the defence select committee at the time of the affair, which the home secretary’s lawyer said would have made him of particular interest to the Kremlin.

Responding to Ms Zatuliveter’s comment that he was just a backbench MP, he said: “He’s not just a backbench MP, though, is he? He’s a member of the defence select committee.”

The Siac panel also heard the Russian had a string of relationships, mostly sexual, with unidentified officials from European countries when she acted as a chaperone to delegates attending conferences in Russia.

Not George Smiley

Tim Owen QC, representing Miss Zatuliveter, told the tribunal the MI5 spooks investigating his client were more like the bumbling Inspector Clouseau than George Smiley, John Le Carre‘s fictional spymaster.

An expert witness for the defence, Nicholas Fielding, told the Siac panel the UK security service case against Ms Zatuliveter was more a “risk assessment” than an “evidence-based case”.

Discussing the elements that made up the case, the defence expert on Russia said: “It runs the risk of appearing to be like a bunch of drunks walking down the road.

“By adding bits to it, it may seem that it adds strength, but if one thing doesn’t add up, you run the risk of making the whole thing collapse.”